(1) Field of the Invention
This invention relates to an organic electroluminescent device comprising a pair of electrodes and an organic film disposed therebetween.
(2) Related Arts
Electroluminescent (hereinafter referred to as EL) devices are divided between inorganic and organic, the latter being very hopeful as new display devices with the advantages of managing with low voltage and easily producing any desired emitting colors.
Of such organic EL devices, the main stream of the present research is one whose structure is called "DH structure", which is described in C. Adachi et. al., J.J.A.P. Vol. 27, No. 2, L269 (1988).
An EL device having typical DH structure comprises, as shown in FIG. 1, a glass substrate 61, a hole injection electrode 62, an organic film consisting of three layers: an organic hole transport layer 63, an organic emitting layer 64 and an organic electron transport layer 65, and an electron injection electrode 66 layered in this order.
Other organic EL devices have either so-called "SH-A" structure having no organic electron transport layer or "SH-B" structure having no organic hole transport layer. The main stream of the present research among them is an organic EL device having the SH-A structure comprising an organic hole transport layer and an organic emitting layer as the organic film between a positive and negative electrodes, which is described in C. W. Tang., et. al., Appl. Phsy. Lett. Vol. 51, No. 12,913 (1987).
These organic EL devices having the above-mentioned structures have the disadvantage of being deteriorated or flaked, while they are being used for hours, by moisture or oxygen in the air. The air intrudes from the border between an exposed area of the organic film and the edge of the adjacent electrode. As a result, non-emitting area in the EL device expands, not maintaining the designed emitting area any more. This problem seriously matters to an apparatus demanding highly accurate resolution such as dot matrix display.
One of the conventional methods to avoid such deterioration of the EL devices caused by moisture or oxygen in the air is to coat the entire EL device with a protecting film made from resin such as acrylic.
However, this method has the following problems:
The resin does not always adhere to the entire surface of the EL device perfectly, thereby leaving slight room between the resin film and the surface of the EL device, through which the air might intrude.
The EL device that has been produced in the vacuum must be put out in the atmosphere to execute such coating process of resin. During the execution, the EL device is exposed to moisture and oxygen, which might spoil the protecting film under formation. Thus the EL device begins to be subjected to damage as soon as it is made up.